How to make my top level root site a "Modern team site"?

So after a lot of confusion and experimentation, what I ended up doing was enabling this tenant to receive the "targeted release" 365 features.  Then I was able try the "new sharepoint admin center preview".  From here, I could create a new top level site (not sure if its still a site collection?)  (such as domain.sharepoint.com/sites/MyNewSite) - which appears to be the new 365 Groups-based site - though it doesn't show up under the old SharePoint Admin Center as a site collection.  And I could use the new modern team site or communication site templates.

But then in the new admin center, there were no controls to turn off external sharing for that site.  I then found powershell commands to disable external sharing for one of the sites.

So as far as I can tell, I've now ended up with 2 new top level sites using new modern team site templates and have one with external sharing enabled and one with external sharing disabled.   Took a lot of researching and jumping through hoops but I feel like I'm in a good situation to start building out the sites now.

Jason Hartman